This invention relates to a packaging bag for a microwave oven capable of heating to cook an article to be cooked, such as ingredients in Chinese foods and curry by the oven.
When a packaged article to be cooked, which generate vapor upon heating, is heated by a microwave oven, there is a fear of bursting of the package by the increase of internal pressure. Accordingly, the heating of the packaged article is, in general, carried out by immersing it in boiling coater.
In order to cook the packaged article by a microwave oven, it is necessary to provide a means for the escape of vapor without liquid escape, such as the attachment of a nonreturn valve or partial cutting of the package by a scissors immediately before cooking.
However, the nonreturn valve has not been spread because of having a problem in incomplete working and expensiveness. There is also a possibility that a liquid portion of the article boils over.
Japanese Patent KOKAI 9-27218 discloses a packaging bag for microwave heating made of a laminated film composed of a heat-resistant thermoplastic resin outer layer, a sealant inner layer interposing a releasing agent layer in lines therebetween, as shown in FIGS. 27, 28. Both ends of the bag are heat-sealed, and during beating by a microwave oven, the inner layer is ruptured at the heat-sealed portion by the increase of internal pressure to release vapor through the ruptured portion. However, this bag still has a problem of not forming a vapor passage smoothly and moreover liquid escape problem.